FIRST EVENING DISCOURSE. 729 
time that Derby wishes to speak to Rugby, then the receiving station at Notting- 
ham would receive both the message from Leicester which it should receive and 
the message from Derby which it was not required to receive. 
To get over this difficulty, known as ‘ interference,’ a large number of devices 
have been patented. The most successful in practice is syntony, or tuning: in this 
method each station has allotted to it one definite frequency or tune, and the 
apparatus is so arranged at each station that it will only be affected by messages 
which are radiated by other stations on its own frequency or tune, and not by any 
other radiations. To take a musical analogy, supposing I had somebody who was 
either deaf to all notes of the piano except, say, the middle ‘C,’ or had such a 
musical ability that he could tell at once when I struck the middle ‘C’; then I 
could transmit to that person a message in the ordinary Morse code by playing on 
the middle ‘C,’ and that person, whom I shall call Mr. C, would not take any notice 
of the fact that I might also be playing on the notes D, K, F, G, &c., but Mr. C 
would confine his attention entirely to what is being done with the middle ‘ C.’ 
It is conceivable that I might find a series of persons or train them so that they 
could each pick out and hear one note only of the piano, irrespective of what was 
being played on the other notes or of any other noises that were taking place. 
Taking an ordinary seven-octave piano and neglecting for a moment the black 
notes, this would give me fifty-six distinct notes on which I could transmit 
messages ; so that, transmitting from Leicester, I might send messages simul- 
taneously to fifty-six different towns. 
The number of possible simultaneous messages depends on the number of octaves 
there are on the piano used, and on how close together the different notes are which 
can be used without producing confusion. For instance, it might be quite easy to 
train someone to distinguish with certainty between ‘C’ and ‘ E,’ and pick out 
signals on ‘C’ at the same time that signals are being sent on ‘EK.’ It is certainly 
more difficult to do this with two notes that are closer together, say ‘C’ and ‘ D,’ 
and still more difficult if the half-tones are used as well. The problem, therefore, 
in wireless telegraphy is to arrange the receiving apparatus so that it can hear, or 
perhaps I should say, more accurately, so that it can only see, notes of one definite 
frequency or pitch, and not be affected by any other notes, even though of but 
slightly different pitch. Another requirement to obtain good working is that we 
should use as little power as possible at our transmitting station consistent with 
obtaining enough power in our receiving instruments to work them with certainty. 
I have a mechanical model to illustrate how we are able to make our receiving 
instruments very sensitive to one frequency and only slightly affected by fre- 
quencies which differ but slightly from its proper frequency. 
The transmitter in the model consists of a disc that can be rotated slowly 
at any speed I like, with a pin fixed eccentrically on its face. This pin can be 
connected to a vertical wire which moves up and down as the disc rotates. I shall 
assume that the movement of this wire corresponds with the movement of the 
electricity in the vertical conductor. As a receiving apparatus I have a pendulum, 
and representing the ether between the transmitter and receiver I have an elastic 
thread connecting the pin in the disc to the pendulum. 
When I set the disc rotating slowly the elastic thread is alternately stretched 
out and relaxed, and the pendulum is a little affected. If I gradually increase the 
speed of the disc at one definite speed it will be found that the pendulum is set 
into violent oscillation, and by observation it will be found that when this is the 
case the dise makes one complete revolution in exactly the same time that the 
pendulum would make one complete swing if left to itself ; that is to say, that 
the disc and the pendulum make the same number of swings per second or have 
the same frequency; in music they would be said to be in tune with each other. 
If instead of allowing the disc to rotate continuously I allow it to make only 
half a dozen revolutions, then the pendulum will be affected, but much less 
strongly. The greater the number of revolutions the disc makes up to a certain 
maximum number the more the pendulum will be caused to swing. 
Instead of starting and stopping the disc I can keep the disc rotating and start 
and stop the pulls on the elastic thread by moving the pin in the face of the dise 
